For the production of thermoplastic plastics materials, it is often necessary to remove residual monomers from the starting plastics material. If, for example, a packaging film for foodstuffs is to be extruded from polystyrene, the proportion of residual monomers, which are harmful to health, may not exceed a predetermined limit value for reasons stipulated in regulations governing foodstuffs and, in consequence, for medical reasons. Since the polystyrene, which is usually to be fed into an extruder, has a proportion of up to 100,000 ppm monomeric styrene, a highly effective degassing arrangement is necessary in order to be able to achieve, in the product, the limit value for residual monomers, which is lower by approximately two powers of ten.
For the degassing of such plastics materials, U.S. Pat. No. 4,094,942 discloses a single-screw extruder for degassing thermoplastic plastics material. In this extruder, in a portion disposed downstream of the feed section, the extruder screw has two pressure restrictors, which are spaced from a screw section producing a mixing effect. Downstream of the first pressure restrictor, the extruder has an injection opening for a stripping agent, which is injectable into the extruded material and is miscible with the molten mass of plastics material in the extruder. In a degassing section disposed downstream of the mixing section, this stripping agent (generally water) then ensures that the molten mass foams-up in the event of a pressure reduction. The increase in size of the free surface of the molten mass, caused by the foam bubbles, effects a degassing of the molten mass which is sufficient for wide ranges of application.
It is additionally known from U.S. Pat. No. 3,613,160 to provide extruders with throttle components, whereby the conveyance of extruded material in the extruder may be variably controlled externally. For this purpose, according to this publication, a substantially cylindrical component is disposed on the screw shaft of the extruder screw, said component rotating jointly with the screw and completely blocking the processing chamber downstream. In the region of this cylindrical component, two throttle pins each extend from externally through the extruder housing radially into an axially oriented overflow conduit, which is incorporated into the internal wall of the extruder housing.
When the throttle pins are retracted, a portion of the extruded material situated upstream of the cylindrical component may pass through these conduits to the downstream section of the extruder. This flow of extruded material can be controlled by the insertion of the throttle pins into these overflow conduits to different depths.
The proportions of residual monomers in the product, which are achievable with such single-screw extruders, however, are not sufficient for many fields of application, and more especially they are not sufficient when the plastics material products come into direct contact, for example, with foodstuffs.
In addition, European Patent Specification No. 0 102 400 belonging to the assignee of the present application discloses that, when degassing rubber mixtures, there are problems in maintaining as pressureless a conveyance as possible of the extruded material in the degassing section, when the mixing section of the extruder operates at a high rate of conveyance and, in addition, the extrusion tool disposed downstream of the extruder produces a high tool counterpressure.
To solve this problem, this publication proposes to increase the diameter of the extruder in the degassing and discharge sections by 10 to 30% compared with the diameter in the mixing section. This arrangement makes available considerably more volume in the processing chamber of the extruder for the extruded material, with the result that, even at a high rate of conveyance and with considerable tool counterpressure, degassing may be effected in an optimum manner.
Nevertheless, however, this extruder is disadvantageous, in that it is only utilizable for a comparatively narrow range of rubber mixtures and tool counterpressures. In the event of the occurrence of greater deviations from the starting value of the extruder, it is necessary to tolerate a poorer degassing performance and/or less output. This is remedied only by adapting the extruder screw to the other above-mentioned extrusion conditions, which may differ considerably.